Tachyarrhythmias are abnormal heart rhythms characterized by a rapid rate, typically expressed in units of beats per minute (bpm), which can originate in either the ventricles or the atria. Examples of tachyarrhythmias include atrial tachyarrhythmias such as atrial flutter and atrial fibrillation (AF), and ventricular tachyarrhythmias such as ventricular tachycardia (VT), and ventricular fibrillation (VF). In contrast, sinus tachycardia (ST) is a benign rhythm which can also result in an elevated heart rate with rates comparable to atrial and ventricular tachyarrhythmias in some patients. The most dangerous tachyarrhythmias are those that have their origin in the ventricles, namely ventricular tachycardia and ventricular fibrillation. Ventricular rhythms occur when re-entry of a depolarizing wavefront in areas of the ventricular myocardium with different conduction characteristics becomes self-sustaining or when an excitatory focus in the ventricle usurps control of the heart rate from the normal physiological pacemaker of the heart, the sino-atrial node. The result is rapid activation of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram (ECG) because they do not use the specialized conduction system of the ventricles, the depolarization spreading instead from the excitatory focus or point of re-entry directly into the myocardium. In ventricular tachycardia, the ventricles activate rapidly and produce distorted QRS complexes in an ECG. Ventricular fibrillation, on the other hand, occurs when the ventricles depolarize at an even more rapid rate and in a chaotic fashion, resulting in electrogram deflections of constantly changing shape and virtually no effective pumping action.
Implantable cardiac rhythm management devices may be configured to treat both atrial and ventricular tachyarrhythmias with electrical therapy. Devices known as implantable cardioverter/defibrillators (ICDs) deliver an electric shock to the heart which terminates the tachyarrhythmia by depolarizing all of the myocardium simultaneously and rendering it refractory. The most dangerous tachyarrhythmias are ventricular tachycardia and ventricular fibrillation, and ICDs have most commonly been applied in the treatment of those conditions. Both ventricular tachycardia and ventricular fibrillation can be hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death and is usually treated with immediate delivery of a defibrillation shock. Ventricular tachycardia can be treated with either a defibrillation or a cardioversion shock, the latter referring to a shock delivered synchronously with an R wave. Dual chamber ICDs are also capable of detecting atrial tachyarrhythmias, such as atrial fibrillation and atrial flutter, and delivering a cardioversion shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, dual chamber ICDs configured to treat atrial fibrillation offer a number of advantages to certain patients, including convenience and greater efficacy.
Another type of electrical therapy for ventricular tachycardia is anti-tachycardia pacing (ATP). In ATP, the ventricles are competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing the tachycardia. ATP therapy can successfully treat VT, but it is not effective in terminating VF. Modern ICDs incorporate ATP capability so that ATP therapy can be delivered to the heart when a ventricular tachycardia is detected. Although cardioversion/defibrillation will also terminate ventricular tachycardia, it consumes a large amount of stored power from the battery and results in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible. In most ICDs with ATP capability, VF is distinguished from VT using a rate-based criterion so that ATP or shock therapy can be delivered as appropriate, where the heart rate is determined by measurement of the time interval between successive ventricular depolarizations. In a typical device, therapy delivery may be programmably allocated into multiple rate zones, with VF therapy delivered in the highest zone, and VT therapy delivered in one or more lower rate zones. A tachyarrhythmia with a heart rate in the VT zone may be treated with ATP therapy in order to avoid an unnecessary painful shock to the patient, and a defibrillation shock is delivered if the heart rate is in the VF zone or if ATP pacing fails to terminate a tachyarrhythmia in the VT zone.
As aforesaid, VT can be detected when the ventricular rate falls within the VT zone. A rapid ventricular rate in the VT zone, however, is not necessarily due VT but can also result from a tachycardia that originates from “above” the ventricles. Such tachyarrhythmias are referred to as supraventricular tachycardias (SVT's) and include ST, which is a normal rhythm, as well as atrial tachyarrhythmias such as atrial tachycardia (AT) of non-sinus origin, atrial flutter (AFL), AV node reentrant tachycardia (AVNRT), and atrial fibrillation (AF). The normal rhythmic impulse of the heart is first generated in pacemaker tissue known as the sino-atrial (SA) node, spreads throughout the atria causing atrial contraction, and is then conducted to the atrioventricular (AV) node where the impulse is delayed before passing into the ventricles. The ventricles of a normal heart are then electrically stimulated by excitation emanating from the AV node that spreads via specialized conduction pathways. An abnormal rhythm in the atria can thus be transmitted antegradely to the ventricles in a patient whose AV conduction pathway is intact. Such an SVT is characterized by elevated rates in both the atria and the ventricles. Elevated rates in both the atria and ventricles can also occur with VT as well, however, due to retrograde conduction of excitation from the ventricles to the atria. Such retrograde conduction is possible in most people and confounds the discrimination between VT and SVT based upon atrial and ventricular rates alone when both rates are similar, a condition known as a one-to-one or 1:1 tachycardia.
It is desirable for an ICD to differentiate between an SVT and a VT, to ensure delivery of appropriate therapy. Ventricular ATP therapy delivered to treat an SVT will not be effective and potentially could make matters worse by triggering a ventricular arrhythmia. Ventricular shock therapy delivered into SVTs can be perceived as painful and is ineffective in reducing elevated heart rates associated with ST. It is thus important for an ICD to recognize that an elevated ventricular rate is due to an SVT rather than a VT so that ventricular ATP therapy can be withheld and specific therapy for the atrial tachyarrhythmia can be delivered if appropriate. Conversely, because VT is generally a more serious condition, the ICD also needs to detect VT with a high degree of sensitivity so that therapy can be delivered promptly. The present invention is directed toward improvements in methods and apparatus for dealing with this problem.